Using the yeast-two hybrid system, our laboratory has identified a cellular protein, NPI-2, which interacts with the influenza A virus nucleoprotein (NP). NPI-2 encodes an N-terminal truncation of the cellular hnRNP D p40 isoform protein. The hnRNP D isoform proteins have been implicated in both stabilization and destabilization of cellular mRNA. In this application, experiments are proposed to determine the significance of the NP:hnRNP D interaction during influenza virus replication. (1) Experiments will be performed to determine whether an interaction between NP and the hnRNP D protein isoforms can be detected in vivo and in vitro. The sequences responsible for the interaction in each protein will also be mapped. (2) The effect of influenza virus infection and/or NP expression on the localization of the hnRNP D isoform proteins and cellular mRNA stability will also be determined. (3) The significance of the NP:hnRNP D interaction during influenza virus infection will be tested by attempting to block the interaction and assaying the effects on influenza virus infection and cellular mRNA stability.